A main striving force in the development of wireless/cellular communication networks and systems is to provide, apart from many other aspects, increased coverage or support of higher data rate, or a combination of both. At the same time, the cost aspect of building and maintaining the system is of great importance and is expected to become even more so in the future.
Until recently the main topology of wireless networks has been fairly unchanged, including the three existing generations of cellular networks. The topology characterized by the cellular architecture with the fixed radio base stations and the mobile stations as the transmitting and receiving entities in the networks, wherein a communication typically only involves these two entities. An alternative approach to networks are exemplified by the well-known multihop networks, wherein typically, in a wireless scenario, a communication involves a plurality of transmitting and receiving entities in a relaying configuration. Such systems offer possibilities of significantly reduced path loss between communicating (relay) entities, which may benefit the end-to-end (ETE) users.
Attention has recently been given to another type of topology that has many features and advantages in common with the multihop networks but is limited to relaying in only two (or a few) hop. In contrast to multihop networks, aforementioned topology exploits aspects of parallelism and also adopts themes from advanced antenna systems. These networks, utilizing the new type of topology, have cooperation among multiple stations as a common denominator. In recent research literature, it goes under several names, such as cooperative relaying, cooperative diversity, cooperative coding, virtual antenna arrays, etc. In the present application the terms “cooperative relaying” and “cooperative schemes/methods” is meant to encompass all systems and networks utilizing cooperation among multiple stations and the schemes/methods used in these systems, respectively. A comprehensive overview of cooperative communication schemes is given in [1]. Various formats of a relayed signal may be deployed. A signal may be decoded, re-modulated and forwarded, or alternatively simply amplified and forwarded. The former is known as decode-and-forward or regenerative relaying, whereas the latter is known as amplify-and-forward, or non-regenerative relaying. Both regenerative and non-regenerative relaying is well known, e.g. by traditional multihopping and repeater solutions respectively. Various aspects of the two approaches are addressed in [2]. The relays may forward the signal in essentially two ways; relaying the signal on the same resource or changing to another channel, e.g. in time or frequency. In the first case, a challenge is to overcome the coupling between the relay transmission and reception. This may be handled by using two antennas, and interference cancellation techniques. In the second case, the relay may simply receive the signal, and then forward it in the next slot or alternatively on another frequency band concurrently as receiving the signal.
The general benefits of cooperative relaying in wireless communication includes higher data rates, reduced outage (due to different forms of diversity), increased battery life, and extended coverage.
Various schemes and topologies utilizing cooperative relaying has been suggested, as theoretical models within the area of information theory, as suggestions for actual networks and in a few cases as laboratory test systems, for example. Examples are found in [1] pages 37-39, 41-44. The various cooperation schemes may be divided based on which entities have data to send, to which and which entities that cooperates. In FIGS. 1a-f (prior art) different topologies are schematically illustrated, showing where traffic is generated, which entity being the receiver and the path for radio transmissions.
The classical relay channel, illustrated in FIG. 1a, consists of a source that wishes to communicate with a destination through the use of relays. The relay receives the signal transmitted by the source through a noisy channel, processes it and forwards it to the destination. The destination observes a superposition of the source and the relay transmission. The relay does not have any information to send; hence the goal of the relay is to maximize the total rate of information flow from the source to the destination. The classical relay channel has been studied in [1], [7] and in [3] where receiver diversity was incorporated in the latter. The classical relay channel, in its three-station form, does not exploit multiple relay stations at all, and hence does not provide the advantages stated above.
A more promising approach, parallel relay channel, is schematically illustrated in FIG. 1b, wherein a wireless systems employing repeaters (such as cellular basestation with supporting repeaters) with overlapping coverage, a receiver may benefit of using super-positioned signals received from multiple repeaters. This is something that happens automatically in systems when repeaters are located closely and transmit with sufficiently large power. Recently, information theoretical studies have addressed this case, for example by Schein, [4] and [5], who suggest the use of coherent combining based cooperative relaying between a single sender and a single receiver using two intermediate relays. The study is purely an information theoretical analysis, limited to only two relay stations, and lacks the means and mechanisms to make the method practically feasible.
The concept of Multiple-access Channel with Relaying (a.k.a. as Multiple access channels with generalized feedback), schematically illustrated in FIG. 1c, has recently been investigated. The concept involves that two users cooperate, i.e. exchange the information each wants to transmit, and subsequently each user sends not just its own information but also the other users information to one receiver. The benefit in doing so is that cooperation provides diversity gain. There are essentially two schemes that have been investigated; cooperative diversity and coded cooperative diversity. Studies are reported in [1], for example. With respect to diversity, various forms has bee suggested, such as Alamouti diversity, receiver diversity, coherent combining based diversity. Typically the investigated schemes and topologies rely on decoding data prior to transmission. This further means that stations has to be closely located to cooperate, and therefore exclude cooperation with more distant relays, as well as the large number of potential relays if a large scale group could be formed. An additional shortcoming of those schemes is that is fairly unlikely having closely located and concurrently transmitting stations. These shortcomings indicates that the investigated topology are of less practical interest. The broadcast channel with relaying, illustrated in FIG. 1d, is essentially the reverse of the topology depicted in FIG. 1c, and therefore shares the same severe shortcomings.
A further extension of the topology depicted in FIG. 1c is the so-called interference channel with relaying, which is illustrated in FIG. 1e, wherein two receivers are considered. This has e.g. been studied in [8] and [1] but without cooperation between the receivers, and hence not exploiting the possibilities possibly afforded by cooperative relaying.
Another reported topology, schematically illustrated in FIG. 1f, is sometimes referred to as Virtual Antenna Array Channel, and described in for example [9]. In this concept, significant bandwidth expansion between a communicating station and adjacent relay nodes is assumed, and hence non-interfering signals can be transferred over orthogonal resources that allows for phase and amplitude information to be retained. With this architecture, MIMO (Multiple Input Multiple Output) communication (but also other space-time coding methods) is enabled with a single antenna receiver. The topology may equivalently be used for transmission. A general assumption is that relay stations are close to the receiver (or transmitter). This limits the probability to find a relay as well as the total number of possible relays that may be used. A significant practical limitation is that very large bandwidth expansion is needed to relay signals over non-interfering channels to the receiver for processing.
As realized by the skilled in the art real system implementations utilizing cooperative relaying needs control mechanisms for controlling the involved relay stations. The need of control arises primarily due to the mobility of the mobile stations and resulting topology changes and may for example include relay activation and deactivation. The need for control mechanisms is schematically illustrated in FIG. 2, wherein a moving mobile station 220 is communicating via the relay stations 215:1 and 215:2 at time T1 and the relay stations 215:2, 215:3 and 215:4 at time T2.
The control procedures are not fully described in the prior art mentioned above. However, it is indicated, for example in the case described with reference to FIG. 1f, that control messages are exchanged to and from the relay stations directed both to and from the base station and the users. Similar control structure is also disclosed in [11], wherein at least one control terminal is identified, which instructs the relay stations to receive and relay data.
The proposed control mechanism may cause the amount of control data to be excessive, especially when topology changes frequently due to that the mobile user moves fast and has to control the relays transmit parameters (e.g. power) or change relays frequently. In addition, even if topology does not change, the changes in radio propagation can be considerable and dictate fast control message exchange towards the relays. An excessive control signalling consumes radio resources that preferably could have been used for transmitting data.
A further problem that is not addressed in the prior art is how to employ session or user centric control of the relays when multiple receivers are present as they may have conflicting optimal relay configurations and parameters settings. The optimality may differ with respect to which relay is active, transmit power levels, channel assignments, space time coding options and phase adjustment etcetera used. A situation which may give arise to conflicting relay configurations and parameters settings is schematically illustrated in FIG. 3, wherein two mobile stations 320:1 and 320:2 are both communicating partly via the same relay station 315:2. The relay station 315:2 may in this scenario experience conflicting demands from the mobile stations 320:1 and 320:2. The optimization problem of finding an optimal, or close to optimal, configuration for a set of users, will, even if only a few users with potentially conflicting optimal configuration are considered, soon become very complicated and time consuming, or even in practise unmanageable.
Thus, it is in the art demonstrated that cooperative relaying has great potentials in providing for example high capacity and flexibility. However, the in the prior art proposed control mechanisms do not represent solutions that are possible to implement in realistic large-scale networks and do not take full advantage of the anticipated advantages of a network with cooperative relaying.